The U.S. Pat. No. 3,410,135 discloses a force measuring device where at the bottom of a potlike base member an elastomeric block is arranged having a slightly raised peripheral lip. A force introduction piston having a remarkably smaller diameter than the cylindrical interior of the base member fits with its lower face edge into the recess formed by the raised lip. The gap between the peripheral surface of the piston and the cylindrical inner surface of the base member is relatively large and of such a width that upon application of a force no elastomeric material is squeezed out of the base member by the piston. In view of the relatively large gap the piston may assume a certain oblique position with respect to the cylindrical interior on the base member without engaging the surface of the latter. In order to limit such an oblique position of the piston a resilient washer may be provided in the upper region of the annular gap between the piston and the interior surface of the base member. There is no connection between the elastomeric material and the metallic surfaces of the base member and the piston in contact with the elastic material. Without the washer considerable lateral forces will be transmitted to the elastomeric material and cause misreadings of the force measured. With the washer provided a new member of friction is introduced which in turn deteriorates the measuring results. Since the annular gap is relatively wide elastomeric material will be pressed into it at least upon application of large forces which gives rise to further inaccuracies of the measurement.
The U.S. Pat. No. 4,175,429 discloses a force measuring device comprising a pressure sensitive metal wire embedded in block of vulcanized silicon rubber provided at the bottom of a cylindrical recess in a base member and compressible by means of a movable piston acting as a force introduction member onto the silicon rubber. A considerable space is allowed between the interior cylindrical wall of the base member and the peripheral surface of the piston. In order that a clearance may be maintained as large as possible a sealing ring is provided at the edge of the lower face surface of the piston in contact with the silicon rubber. The sealing ring is in contact with the cylindrical surface of the interior of the base member which results in a considerable friction affecting the accuracy of the force measuring device. Furthermore, any lateral force component of a force applied at an oblique angle onto th force introduction members acts at least partially onto the elastomeric material which again deteriorates the accuracy of measurement.
The German Patent Publication No. 1,932,899 discloses a force measuring device comprising two flat elastic members and material of high electric resistance which is arranged in form of lines or areas between the elastic members. The material is subjected to mechanical strain only in normal direction to the layer and changes its resistance dependent on the mechanical strain. With the known force measuring device force introduction is difficult. Furthermore, the manufacture is complex and the measurement accuracy insufficient for many applications. Also, the known device is not adapted for measuring very high forces.
The German Patent Publication No. 3,011,266 discloses a pressure measuring device for arealike pressure distributions. The device comprises a mat of elastic material changing its electrical conductivity dependent on pressure exertion. On each main surface a pattern of electrodes is provided the two patterns crossing each other the points of crossing defining a measuring point; at each measuring point a metal plate is secured to the mat the size of the metal plate corresponding the overlying areas of the electrodes. The known device is not adapted for very large loads and the accuracy is low. There is little resistance against wear.